The present invention relates to a method for emitting very low frequency, high power acoustic waves, and providing transducers for producing these waves. The technical field of the invention is the realization of electro-acoustic transducers for emitting acoustic waves immersed in a liquid. The principal application of the invention relates to the possibility of emitting very low frequency and high power acoustic waves, whatever the immersion depth is.
In particular, known for such low frequency underwater acoustic waves, are the submersible electro-acoustic transducers called flextensional transducers; they comprise one or more electro-acoustic motors, generally piezoelectric motors, located according to a same interior plane or as a peripheral ring of an envelope or flexible shell. This shell, when put in contact with water, constitutes the emitting surface of the acoustic waves. These transducers convert the shell oscillations generated in the plane where the electro-acoustic motors are located, and due to the dilation-compression effects, they are converted into oscillations perpendicular to this plane and thus in plane deflection, hence their name: flextensional transducers. They make it possible to amplify the motor oscillation amplitude by a factor of 3 and even 4 and thus to obtain very low frequencies.
It is recalled that the known flextensional transducers, can be grouped in five classes depending on their general shell shape:
Class I groups transducers with a shell in ellipsoid of revolution around an axis and a single electro-acoustic motor, which may be piezoelectric or magnetosostrictive, and is coaxial with the revolution axis of the shell, and which is mechanically and acoustically coupled by both extremities with both portions of the shell surfaces located on the axis of revolution.
Class II groups the transducers with a disk-shaped or revolution ring-shaped shell around the disk or ring axis. These transducers comprise several piezoelectric motors, radially arranged around the revolution axis of the shell and mechanically and acoustically coupled to the revolution axis, either by one of their extremities in the case of a disk shape, with the other extremity resting on a central pillar, or by both extremities in the case of a ring shape;
Class III groups the transducers whose shell has two bulges at both extremities and whose general shape is of the bone or diabolo type;
Class IV groups the transducers whose shell is an elongated cylinder, whose straight cross section is elliptical or is a closed curve with a throttler in its central part. The transducers of this class comprise several electro-acoustic motors parallel to each other and whose axes are located in transverse planes perpendicular to the cylinder generatrix and following the direction of the largest of the section axis: both extremities of each motor are mechanically and acoustically coupled to the cylindrical shell;
Class V groups the transducers whose shell is a combination of the shape of those of Class I and those of Class IV, or those of Class II; and which can be qualified as "oyster" shape: in fact, it is an ellipsoid type shape, possibly of revolution, but flattened and not elongated as in Class I, or a very flattened spheroid, or even a toric generated by a circumference comprising a rectilinear side around which it rotates and which constitutes its axis.
The present invention is related more particularly, but not exclusively, to flextensional transducers of Classes IV and V above.
All the flextensional transducers have many advantages for the emission of low and very low frequency waves, nevertheless they have also disadvantages as they require the shells or envelopes, to be either closed and water-tight, or open and non water-tight,in which case special devices are now used to limit the effects of this non-watertightness.
Indeed, the acoustic waves to be emitted in low frequencies must be provoked by the vibrations of external shells alone, and no other wave induced by internal shell vibrations and electro-acoustic motors must escape from the inside of the latter in order not to interfere with the low frequency emission. Thus, as a first alternative, the shell may be closed and filled with gas, and it must then resist the immersion pressure. However, the use of such transducers in low depths is limited, for if the shell resistance must be increased to withstand great depth pressure, the elasticity is decreased and therefore the possibility of emission increases. To avoid increasing this resistance and to keep this elasticity, specific devices have been developed, some of which are disclosed in various patents.
Indeed, all these devices consist of using the possibility of compensating the external pressure by increasing the internal pressure, and in various ways depending on the inventions, in order to avoid a water-tight box to withstand the efforts of resistance to external pressure. The following may be mentioned:
French patent application FR 2 634 292, called "method and device to maintain the pressure of the gas contained in an immersed chamber the pressure in balance with the external pressure", consists in associating with an immersed chamber, such as the box of a piezoelectric transducer, several bottles which contain a deformable pocket, preinflated at various pressures, and thus making it possible to compensate the hydrostatic pressure at various depths of immersion;
French patent application FR 2 665 814, relating to "electro-acoustic transducers intended for immersion", includes an automatic compensation system of the immersion pressure, thanks to chambers filled with gas and reduced volumes, in order to compensate only the axial loads exerted on the ceramic central pillar of the transducer.
Other patent applications could be mentioned which use pneumatic systems of compensation of the external immersion pressure, but comprising all the mechanical means fairly voluminous gas and/or complicated supply or storage means.
Another known alternative consists in having a non water-tight shell, and filling it with liquid to eliminate the here-above compensation devices:
by using a fluid different from the ambient medium and by installing a separating membrane which also acts as a flexible diaphragm compensating the differences of internal volume due to the compressibility of the internal fluid, generally oil. This is disclosed in French patent application FR 2 671 927, relating to "directive electroacoustic transducers and manufacturing methods and devices"; but in the present application to very low frequency transducers of the flextensional type, it is then impossible to eliminate the waves induced by the internal vibrations, which is not the aim sought after; or PA1 by using the liquid of the ambient medium. The present invention belongs to this last category, in which, presently, in order to solve both the problem of behaviour in immersion, and that of the internal waves, communication orifices are made in the transducer shell, having dimensions so that the fundamental vibration frequency of the whole is to be close to the Helmholtz frequency of the cavity and, moreover, depending on the case, various devices are added in these orifices and/or the cavity formed by the shell to adjust both frequencies and obtain the required effect. PA1 the liquid is allowed to enter the entire inside cavity delimited by this envelope by means of any orifice communicating with the outside; PA1 a baffle, preferably made up of alveolar material resistant to the immersion pressure, is placed in the cavity; it occupies most of the cavity volume and its external surface is at a constant distance from that of the envelope; PA1 the baffle is interdependent to the assembly device of the acoustic motors; PA1 the motors are requested to emit the very low frequency and high power acoustic waves.
However, even in this case, the emission is disturbed, especially in the range of very low frequencies, and moreover, contrary to the objectives of the present invention, the emission level is lowered, thus limiting the possibility of emitting at high power.